Scrabble champion's first rule (4)
I believe the answer is:
claw
'scrabble' is the definition.
(I've seen this in another clue)
'champion's first rule' is the wordplay.
'champion's first' becomes 'c' (1st letter of 'champion').
'rule' becomes 'law' (law is a kind of rule).
'c'+'law'='CLAW'
(Other definitions for claw that I've seen before include "Hook on the end of toe" , "Sharp curved toe of a bird" , "Lobster's grasping structure" , "Bird's foot" , "Rake" .)